The semiconductor industry has experienced rapid growth due to improvements in the integration density of a variety of electronic components (e.g., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from shrinking the semiconductor process node (e.g., shrink the process node towards the sub-20 nm node). As semiconductor devices are scaled down, new techniques are needed to maintain the electronic components' performance from one generation to the next. For example, high breakdown voltage transistors are desirable for high power applications.
As semiconductor technologies evolve, metal oxide semiconductor (MOS) transistors have been widely used in today's integrated circuits. MOS transistors are voltage controlled device. When a control voltage is applied to the gate a MOS transistor and the control voltage is greater than the threshold of the MOS transistor, a conductive channel is established between the drain and the source of the MOS transistor. As a result, a current flows between the drain and the source of the MOS transistor. On the other hand, when the control voltage is less than the threshold of the MOS transistor, the MOS transistor is turned off accordingly.
MOS transistors may include two major categories. One is n-channel MOS transistors; the other is p-channel MOS transistors. According to the structure difference, MOS transistors can be further divided into three sub-categories, planar MOS transistors, lateral double diffused MOS transistors and vertical double diffused MOS transistors. In comparison with other MOS transistors, the lateral double diffused MOS transistors are capable of delivering more current per unit area while maintaining a high breakdown voltage. Lateral double diffused MOS transistors may be alternatively referred to as high voltage MOS transistors.
In order to improve the reliability of high voltage MOS transistors, a variety of qualification tests may be performed on a high voltage MOS transistor. For example, a high temperature reverse bias (HTRB) test is performed to accelerate failure mechanisms. It is done by applying about 80%-100% of the maximum voltage to which the high voltage MOS transistor is specified while stressing the high voltage MOS transistor at a temperature range from about 125 degrees to 200 degrees up to about 168 hours. Such a high temperature test helps to accelerate failure mechanisms so as to simulate a test conducted at a lower temperature for a greater length of time. A pressure cooker test (PCT) is another qualification test normally performed on a high voltage MOS transistor. It may be performed at 150 degrees and 2-atm pressure. The PCT helps to test the high voltage MOS transistor's package moisture resistance.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.